The present invention relates to a DC-DC converter, and more particularly, to a synchronous rectifier type DC-DC converter used as a power source for electronic devices and a controller for such DC-DC converter.
The recent increase in the operational frequency speed of a CPU for electronic devices has resulted in an increase in the power source current. This, in turn, has increased the power of a synchronous rectifier type DC-DC converter, which is used as a power source of the CPU. In a DC-DC converter, a main switching device and a synchronous switching device, which are connected in series, are alternately activated and deactivated to supply a constant voltage to a load. When these switching devices are short-circuited, an amount of current greater than that during normal operation may flow toward other externally connected devices and damage the externally connected devices. Accordingly, it is required that the externally connected devices be protected.
FIG. 1 is a schematic circuit diagram of a prior art DC-DC converter 1. The DC-DC converter 1 includes a controller 2, which is formed as a circuit on a single semiconductor substrate, and a plurality of externally connected devices, that is, a main switching device 3, a synchronous switching device 4, a choke coil 5, a flyback diode 6, and a smoothing capacitor 7.
The controller 2 provides the main switching device 3 with a first drive signal SG1. The switching device 3 is an enhancement n-channel MOS transistor, which gate is provided with the drive signal SG1. The drain of the switching device 3 is supplied with power source voltage Vi from a battery E. The source of the switching device 3 is connected to the synchronous switching device 4.
The synchronous switching device 4 is an enhancement n-channel MOS transistor having a drain connected to the source of the main switching device 3. The gate of the synchronous switching device 4 is provided with a second drive signal SG2 from the controller 2 and the source is connected to the ground GND.
The source of the main switching device 3 is connected to an output terminal To via the choke coil 5, and also to the cathode of the flyback diode 6, which anode is connected to the ground GND.
The output terminal To is connected to the ground GND via the smoothing capacitor 7 and to a load (not shown), such as a CPU. The output terminal To provides the load with an output voltage Vo and returns the output voltage Vo to the controller 2.
With reference to FIG. 2, the controller 2 provides the switching devices 3, 4 respectively with first and second drive signals SG1, SG2, which are essentially complementary signals. This alternately activates and deactivates the main switching device 3 and the synchronous switching device 4. The switching of the main switching device 3 fluctuates the voltage VS at a node N1 between the two switching devices 3, 4 in a pulse-like manner. The voltage VS is smoothed by the choke coil 5 and the smoothing capacitor 7 to generate a predetermined output voltage Vo.
The controller 2 compares the returned output voltage Vo with a reference voltage to vary the duty ratio of the first and second drive signals SG1, SG2. This enables the DC-DC converter 1 to substantially match the output voltage Vo with a set voltage.
When the main switching device 3 is deactivated, the synchronous switching device 4 is activated. This maintains the voltage drop of the voltage VS, which is caused by the forward voltage VD of the flyback diode 6, at substantially zero volts. Thus, the synchronous switching device 4 suppresses power consumption by the flyback diode 6 and prevents the smoothing efficiency from being decreased. This improves the efficiency of the DC-DC converter 1. Thus, the synchronous rectifier DC-DC converter 1 is used in equipment that requires a large output current.
However, when a short-circuit occurs between the drain and the source of the main switching device 3, the output voltage Vo becomes higher than the set voltage. Since such short-circuit results in an abnormal output, a protection measure has been provided in the prior art.
When a short-circuit occurs between the gate and source of the synchronous switching device 4, the switching device 4 is deactivated. Thus, the DC-DC converter 1 operates as a normal DC-DC converter, which does not have the synchronous rectifying function, and the output voltage is almost the same as that during a normal state. However, the rating of each of the devices 3-7 is set as for a synchronous rectifier type DC-DC converter. Thus, the load applied to the main switching device 3 and the flyback diode 6 increases, which may, in turn, produce heat and damage the devices. Such problem is especially prominent in a DC-DC converter having a large output current.